When there is a need to supply fluid flow rates to a few hundred to a several tens of thousands of m.sup.3 per hour and at a pressure lying in a range from about 100-150 bars to about 300-350 bars, one known technique consists in associating centrifugal compressors with alternating piston compressors or with screw compressors, such that the fluid is initially compressed by the centrifugal compressors, and is subsequently compressed by the piston or screw compressors.
Such a compressor set may, in particular, produce compressed gas for injection into an oil well (on- or off-shore) for the purpose of assisted oil recovery. This known method of exploiting an oil field is characterized, inter alia, by the need to reduce the rate of gas flow as the oil is extracted. This flow rate reduction constitutes a major drawback when the compressed gas is supplied by a set of centrifugal compressors together with piston or screw compressors, since there is then a risk of reaching the "pumping limit" below which very large pressure oscillations occur together with vibrations that are capable of destroying the compressors.
This risk can be avoided by increasing the number of centrifugal compressor stages, but only at the cost of corresponding increases in capital expenditure, and in maintenance and running costs.
Further, the piston or screw compressors which are associated with the centrifugal compressors have drawbacks of their own; they are expensive and bulky, they require frequent and expensive maintenance, and they vibrate and are noisy in operation. It is also necessary to provide step-down gearing so that they can be driven from the same motors that drive the centrifugal compressors.
Preferred embodiments of the present invention provide machines for compressing a fluid and capable of operating in the above-mentioned range of flow rates and pressure, but which avoid or greatly reduce the above-mentioned drawbacks of the prior art.
Such preferred machines are compact, monoblock device that take up little space.
Such preferred machines are additionally much more reliable than prior art machines of equivalent performance.